Inhibition of adenylate cyclase from luteinized rat ovary by monovalent cations: Roles of the stimulatory guanine nucleotide-binding regulatory component and stimulatory hormone receptor

Abstract

Sodium and other monovalent cations (added as chloride salts) inhibited adenylate cyclase of luteinized rat ovary. Sodium chloride (150 m m) inhibited basal enzyme activity by 20%. Sodium chloride inhibition was enhanced to 34–54% under conditions of enzyme stimulation by guanine nucleotides (GTP and its nonhydrolyzable analog 5′-guanylyl imidodiphosphate), fluoride anion, and agonists (ovine luteinizing hormone (oLH) and the β-adrenergic catecholamine isoproterenol) acting at stimulatory receptors linked to adenylate cyclase. Sodium chloride inhibition was dependent on salt concentration over a wide range (25–800 m m) as well as the concentrations of GTP and oLH. Inhibition by NaCl was of rapid onset and appeared to be reversible. The order of inhibitory potency of monovalent cations was Li + > Na + > K +. The role of individual components of adenylate cyclase in the inhibitory action of monovalent cations was examined. Exotoxins of Vibrio cholerae and Bordetella pertussis were used to determine respectively the involvement of the stimulatory and inhibitory guanine nucleotide-binding regulatory components (N s and N i) in NaCl inhibition. Sodium chloride inhibited cholera toxin-activated adenylate cyclase activity by 29%, N i did not appear to mediate cation inhibition of adenylate cyclase because pertussis toxin did not attenuate inhibition by NaCl. Enzyme stimulation by agents (forskolin and Mn 2+) thought to activate the catalytic component directly was not inhibited by NaCl but was instead significantly enhanced. Sodium chloride (150 m m) increased both the K d for high-affinity binding of oLH to 125I-human chorionic gonadotropin binding sites and the K act for oLH stimulation of adenylate cyclase by sevenfold. In contrast, NaCl had no appreciable effect on either isoproterenol binding to (−)-[ 125I]iodopindolol binding sites or the K act for isoproterenol stimulation of adenylate cyclase. The results suggest that in luteinized rat ovary monovalent cations uncouple, or dissociate, N s from the catalytic component and, in a distinct action, reduce gonadotropin receptor affinity for hormone. Dissociation of the inhibitory influence of N i from direct catalytic activation could account for NaCl enhancement of forskolin- and Mn 2+-associated activities. On the basis of these results, the spectrum of divergent stimulatory and inhibitory effects of monovalent cations on adenylate cyclase activities in a variety of tissues may be interpreted in terms of differential enzyme susceptibilities to cation-induced uncoupling of N and catalytic component functions.